Coreless induction furnaces have been used to date as a melt unit in the production of ductile iron. Treatment of the molten metal has always taken place outside of the induction furnace. This method produces a variable temperature condition of the molten metal during treatment in the separate vessel, leading to certain temperature losses, and produces a variable final sulfur level which is in part dependent upon the variable temperature condition. These variable conditions can lead to the generation of scrap castings. Treatment of the metal outside of the furnace, of course, requires the use of a desulfurizing agent or slag as well as the use of an agent for graphitizing the metal.
The coreless induction furnace has never been visualized as a unit for the direct production of ductile iron utilizing a normal furnace operational mode. This may be due in part to the fact that such furnaces have been traditionally lined with silica as dictated by furnace manufacturers seeking to keep costs low; silica is acid in nature and would be attacked by desulfurizing agents which are generally basic in nature if used in the furnace. Thus, silica being an extremely economical lining material and having sufficient structural strength, has been the dominant furnace refractory lining material for a number of years.
The coreless induction furnace has not been employed for the direct production of ductile iron for other reasons, such as: the extreme high cost of low sulphur content charge material, and the extremely stringent process controls that are required even with low sulfur content charge material. Desulfurizing agents, used traditionally for ductile iron treatment, are typically basic in nature, including sodium carbonate, quick-lime (CaO) and calcium carbide. In many cases these desulfurizing agents are mixed with other alkaline materials. Their strong chemical activity has caused the foundry industry to employ vessels which are suitably lined independent of the furnace to resist such chemical activity.
A refractory lining which is more chemically neutral to desulfurizing agents is alumina-chrome refractory material; it has heretofore been used in the steel making process such as evidenced by U.S. Pat. No. 4,039,344. Alumina-chrome refractory means a refractory composition about 30 to about 60 weight percent alumina and about 40 to about 70 weight percent of a chrome comprising material such as chrome ore, chrome magnesia clinker or chrome magnesia brick scrap. Suitable alumina-chrome refractory for this purpose is disclosed in U.S. Pat. No. 4,039,344 which is incorporated herein by reference as a teaching of such material.
In the steel making process, the chemistry of the molten metal is of the following nature:
______________________________________ Typical Metal Chemistry Typical Slag Chemistry Element % Compound % ______________________________________ Carbon .1-1.5 Aluminum Oxide 5 Silicon .1-1.5 Silicon Oxide 26 Manganese .1-1.0 Calcium Oxide 56 Chrome .1-1.0 Magnesium Oxide 10 Sulphur .03-.3 ______________________________________
The use of alumina-chrome refractory with the aforementioned chemistry would not indicate its utility in the environment of a melting facility for a foundry where the molten metal and slag have a different chemistry such as:
______________________________________ Typical Metal Chemistry Typical Slag Chemistry Element % Compound % ______________________________________ Carbon 3.25-4.0 Aluminum Oxide 10 Silicon 2.0-2.5 Silicon Oxide 42.5 Manganese .3-1.0 Calcium Oxide 32.0 Chrome .1-1.0 Magnesium Oxide .5 Sulphur .03-.1 ______________________________________
Accordingly, the use of alumina-chrome refractory in a coreless induction furnace has not been visualized because (a) the thermal expansion characteristics of the material have not been predictable in a different environment, (b) inability to cure the material in a changed environment, and (c) lack of a satisfactory installation method in a coreless induction furnace.
Thus, even though alumina-chrome refractory material is known and strongly basic desulfurizers have been employed heretofore in separate treatment ladles, it has never occurred to the prior art that preparation of a ductile iron can be directly provided in a coreless induction furnace in view of the above problems.